At present, an optical communication system has a tendency to become a more advanced and complicated system. For example, together with higher functionalization of an optical communication system such as a WDM optical communication system, it is demanded to highly functionalize an optical waveguide device (semiconductor device) such as an optical semiconductor device (for example, semiconductor laser or the like) to be used in such an optical communication system as just mentioned.
As one of means for implementing this, it is considered a possible idea to configure an optical waveguide device (optical semiconductor device) as an optical integrated device wherein a plurality of devices are monolithically integrated.
As such an optical integrated device (optical waveguide device) as just mentioned, for example, a modulator integration type DFB laser (for example, EML; Electroabsorptive Modulated laser) is available wherein a DFB (Distributed Feed Back; distribution feedback type) laser normally used as a light source for optical communication and a modulator (for example, EA modulator; Electro-absorption Modulator) for modulating an optical output of the DFB laser are integrated. By using the modulator integration type DFB laser, when compared with a case wherein a laser and a modulator are prepared separately from each other, higher functionalization of an optical waveguide device can be implemented while achieving downsizing and reduction in cost, and in its turn, a highly functionalized WDM (Wavelength Division Multiplexing) optical communication system can be implemented.
Incidentally, in such an optical integrated device (optical waveguide device) as described above, two kinds or more of devices (optical waveguides) formed from materials or in structures different from each other are integrated on the same substrate. Therefore, it is necessary to connect devices (optical waveguides) formed from materials or in structures different from each other.
Where two devices (optical waveguides) whose materials or structures are different from each other are connected, reflection occurs on the connection interface without fail, and there is the possibility that the reflected light may cause degradation of characteristics of the devices.
For example, where a semiconductor optical amplifier is integrated, gain ripples are generated due to unintended interference by the reflected light, resulting in appearance of wavelength variation of the gain. Further, for example, where a laser is integrated, an oscillation state of the laser is destabilized by reflection returning light, and this makes stabilized operation difficult.
Therefore, in an optical waveguide device wherein a plurality of devices are integrated, it is an important subject to suppress reflection on a connection interface along which two devices (optical waveguides) are connected in order to implement stabilized operation without degrading characteristics of the integrated devices.
Here, as a technique for suppressing reflection on a connection interface, techniques disclosed, for example, in Patent Documents 1 and 2 are available.
In Patent Document 1, as shown in a schematic sectional view in FIG. 20, where an optical waveguide 100 having a core layer 100A whose refraction index is n1 and another optical waveguide 101 having a core layer 101A whose refraction index is n2 are connected (n1>n2), the thickness d1 (or width) of the core layer 100A whose refraction index is n1 is set smaller than the thickness d2 (or width) of the core layer 101A whose refraction index is n2 (d2>d1) (in other words, the thickness (or width) of the core layer having a higher refraction index is set to a relatively small thickness) and the equivalent refraction indexes of the optical waveguides 100 and 101 to be connected to each other are set so as to coincide with each other to suppress the reflection.
In this technique, since the reflection index on the connection interface increases as the difference between the equivalent refraction indexes of the optical waveguides 100 and 101 to be connected to each other increases, the equivalent refraction indexes of the optical waveguides 100 and 101 to be connected to each other are made coincide with each other to suppress the reflection.
In Patent Document 2, as shown in a schematic top plan view in FIG. 21, the connection interface between two optical waveguides 102 and 103 are set so as to be directed obliquely with respect to the advancing direction of light (optical waveguide direction) to prevent returning of the reflected light reflected on the connection interface to the optical waveguides thereby to suppress the influence of reflection on the connection interface.
Patent Document 1: Japanese Patent Laid-Open No. 2000-235124
Patent Document 2: Japanese Patent Laid-Open No. HEI 9-197154